Temperatures in Bordeaux are forecast to reach 33C during the day, about 7C above the average high between 1961 and 1990, according to the Reuters Climate Monitor.
"The situation we are dealing with today is the most severe we have ever recorded, the toughest since the Second World War," President Emmanuel Macron, who flew to the area, said on Monday evening.
Fire crews had contained the blaze overnight, but the situation remained difficult, said Nathalie Delattre, senator for Gironde, which covers the city of Bordeaux.
"Everything will depend on how the firefighters on the ground get on," she told BFM TV on Tuesday.
France is going through an unprecedented wildfire season.
The area burned has already surpassed the previous record year of 2022.
The Landes area, southwest of Bordeaux, is covered by pine forests, which become highly flammable when dry.
The fire has burned 42,000 hectares.
Authorities have evacuated about 220,000 people, including holidaymakers and local residents.
In the area of Biscarrosse, a town about 40km south, which was hit by another wildfire last week, the situation has improved and some 15,000 evacuated people were gradually allowed back home in the town and a campsite nearby, the local administration said on X.
"However, (the favourable) development does not mean the risk has been eliminated. Flare-ups remain possible, particularly due to weather conditions (wind, drought, etc)," it said.
Temperatures were set to increase in the region from the afternoon and peak at 37C on Wednesday, weather office Meteo France said.
